Radiocommunication networks were originally developed primarily to provide voice services over circuit-switched networks. The introduction of packet-switched in, for example, the so-called 2.5G and 3G networks enabled network operators to provide data services as well as voice services. Eventually, network architecture will evolve toward all-IP networks which provide both voice and data services. However, network operators have a substantial investment in existing infrastructure and would, therefore, typically prefer to migrate gradually to all-IP network architectures to allow them to extract sufficient value from their investment in existing infrastructures. In order to provide the capabilities needed to support next generation radiocommunication applications, while at the same time using legacy infrastructure, network operators will deploy hybrid networks wherein a next generation radiocommunication system is overlaid onto an existing circuit-switched or packet-switched network as a first step in the transition to an all IP-based network.
One example of such a hybrid network involves an existing 3GPP2 radiocommunication system, such as a high rate packet data (HRPD) system, onto which a next generation “long term evolution” (LTE) system is overlaid. As will be appreciated by those skilled in the art, HRPD systems are sometimes referred to by many different names or acronyms. For example, HRPD systems have sometimes been referred to as “high rate data” (HRD) systems or by reference to their air interface standard promulgated by TIA-EIA, i.e., IS-856. The IS-856 standard, entitled “cdma2000® High Rate Packet Data Air Interface Specification (2000)”, which is available online at www.tiaonline.org, is incorporated here by reference. Additionally, since HRPD systems use a code division multiple access (CDMA) scheme and evolved from CDMA 2000, they are also sometimes referred to as “1×EV-DO” systems which refers to an “EVolution, Data-Only” version of CDMA 2000. Similarly, LTE systems refer to, for example, next generation (4G) wideband CDMA (WCDMA) systems which are intended to provide improved performance. Although not yet standardized, LTE systems will ultimately be designed in accordance with a new version of the UMTS standards, see, e.g., 3GPP TR 25.913 available online at www.3gpp.org. Target performance goals for LTE systems currently include, for example, support for 200 active calls per 5 MHz cell and sub 5 mS latency for small IP packets.
When an LTE system is overlaid onto an HRPD system, various types of inter-system interoperability will become desirable, one of which is handoff or handover. Inter-system handoff refers to, for example, the process whereby a mobile unit, e.g., a cellular telephone, wireless PDA or laptop, which is currently being supported by a first radiocommunication system is transferred to the support of a second radiocommunication system. In the context of this application, as shown conceptually in FIG. 1, an inter-system handoff of interest involves the transfer of communication support of mobile unit 10 from an HRPD access network 20 to an LTE access network 30 or vice versa. Such handoffs may be performed for a variety of reasons. For example, a mobile unit 10 which is currently being served by the HRPD access network 20 may have moved into a geographic area wherein it can be better served by the LTE access network 30. Alternatively, the inter-system handoff may be performed to provide load balancing between the HRPD access network 20 and the LTE access network 30.
Regardless of the particular reason for the handoff, various signaling needs to be performed in order to complete the transfer of support responsibility for the mobile unit 10 from or to the LTE access network 30. Additionally, unless the mobile unit 10 can simultaneously receive signals from both the HRPD access network 20 and the LTE access network 30, the inter-system handoff will be a hard (“break before make”) handoff rather than a soft (“make before break”) handoff. For hard handoffs it is further desirable to minimize the intervening time interval during which the mobile station 10 is not connected to either the HRPD access network 20 or the LTE access network 30. Accordingly the exemplary embodiments described herein address the need for mechanisms to facilitate such inter-system handoffs.